1. Field of the Invention
The present invention relates to a contact type image sensor for image input device of a facsimile machine or the like.
2. Description of the Related Art
A contact type image sensor is a photoelectric conversion sensor for reading an image erectly in the equal magnification by using a sensor section whose size is the same as the document, the sensor being placed close to the document. The features of the contact type image sensor are small in size, free adjustment in the optical system, free maintenance, easy machine designing, and so forth. As various office automation machines with an image input apparatus, such as facsimiles, OCRs, and copy machines, have spread, the contact type image sensor has been widely used.
FIG. 1 shows an example of the structure of a conventional contact type image sensor (see a paper titled "Current Situation of Amorphous Silicon Devices (translated title)" in a symposium of Society of Electrophotography of Japan (issued on May 24, 1985).
The contact type image sensor shown in FIG. 1 comprises a sensor section A and an illumination source B. The sensor section A comprises a transparent glass substrate 1, a common electrode 2, an amorphous silicon layer (hereinafter named the a-Si layer) 3 as a photosensitive material layer, a transparent electrode 4 as a separate electrode, and a protection layer 5, which are layered on the glass substrate 1 one after the other. On the sensor section A made of such parts, 2000 to 3000 photosensitive elements are disposed in an island shape and in pitches of 0.1 to 0.2 mm. Each photosensitive element disposed in the island shape has a light conductive window 6. The illumination source B comprises an LED chip 7 and an LED substrate 8. The light originated from the LED is irradiated to a document S through the glass substrate 1 and the light conductive window 6. The reflected light is entered into the a-Si layer 3 through the transparent electrode 4 as a signal light.
It is preferable to dispose one LED chip 7 in accordance with one light conductive window 6. However, when 2000 to 3000 LED chips 7 are disposed in pitches of 0.1 to 0.2 mm, the image sensor becomes very expensive. Thus, conventionally, the LED chips 7 whose size is 0.3 mm.times.0.3 mm are disposed in pitches of approx. 2.5 mm. However, in this case, to prevent the light from being nonuniformly irradiated to the document, it is necessary to dispose the illumination source B apart from the document S and the sensor section A. Thus, thus far, it was difficult to produce the contact type image sensor in a thin shape and at a low cost.
To solve such problems, a contact type image sensor which uses high frequency driving Electro Luminescence (hereinafter named EL) EL elements as the illumination source B has been known (disclosed in Japanese Patent Laid-Open Publication No. SHO 62-279776).
The aforementioned contact type image sensor is provided with as a sensor section a thin film photo sensitive element, which comprising a common electrode, an a-Si layer, a transparent electrode, a transparent protection layer, and so forth, the thin film photo sensitive element being disposed on one surface of a transparent substrate such as a glass substrate. On the other surface of the transparent substrate, an EL element is disposed as an illumination source, opposed to the sensor section, the EL element comprising a transparent electrode, a light emission layer composed of ZnS:Mn, CaS:Eu, SrS:Ce and so forth, an insulation layer, and a rear surface electrode, which are layered one after the other. The EL element is in a belt shape, where the width is approx. 2 mm and the length is approx. 250 mm, for example. The EL element is disposed so that it covers the entire a-Si layer.
The aforementioned EL element can be equally formed by means of thin film technologies or thick film technologies. Thus, the distribution of the light emission intensity can be kept uniform on the entire surface of the EL element. In addition, since the sensor section and the illumination source can be formed on the same glass substrate, the EL element can be structured in a thin shape.
However, to cause the aforementioned EL element to emit light, a high frequency power is required. Generally, in the image sensor, the intensity of the illumination light should be always constant. Alternatively, in this sensor, the lighting frequency of the electro luminescence light should be remarkably faster than the reading speed of an image. For example, in the G III type facsimile, the driving frequency should be 20 kHz or more, preferably, 40 kHz against the reading speed of 10 ms per line. On the other hand, in a thick film type powder EL element, when the frequency exceeds 5 kHz, the lighting frequency cannot follow the frequency of the power. In addition, this type EL element remarkably heats up and the material is occasionally melted at a particular frequency. On the other hand, although the thin film type EL element can satisfy the aforementioned conditions with respect to the light frequency, it requires a high voltage of approx. 200 V to accomplish the particular luminance as the light source of the image sensor. Thus, this type EL element spends much power and requires a large and complicated power circuit for preventing the reading operation from being affected by a power noise. In addition, since this EL element is driven at a high voltage, the emission ray fluctuates and thereby a reading error tends to occur.
Although the aforementioned Japanese Patent Laid Open Publication No. SHO 62-279776 describes the availability of a plasma element panel and a fluorescent element panel instead of the EL element, the thickness of such substitutes is larger than that of the EL element. Thus, it is difficult to effectively decrease the thickness of the machines. In addition, when such substitutes are used, they require vacuum environment in production. Therefore, when they are used in an application where the ratio of width and depth is high, such as the image sensor, the strength of the panel may be disadvantageously affected.